1. Field of the Invention
The present invention generally relates to a method for producing a metal/ceramic bonding substrate. More specifically, the invention relates to a method for producing a metal/ceramic bonding substrate, wherein a metal member of aluminum or an aluminum alloy is bonded to a ceramic substrate, by cooling and solidifying a molten metal of aluminum or the aluminum alloy injected into a mold in which a ceramic substrate is arranged.
2. Description of the Prior Art
In a conventional power module used for controlling heavy-current for electric vehicles, electric railcars, machine tools and so forth, a metal/ceramic insulating substrate is fixed to one side of a metal plate or compound material called base plate by soldering, and semiconductor chips are fixed to the metal/ceramic insulating substrate by soldering. On the other side (reverse) of the base plate, a radiating fin or cooling jacket of a metal is mounted via a thermal conduction grease by means of screws.
Since the soldering of the base plate and semiconductor chips to the metal/ceramic insulating substrate is carried out by heating, the base plate is easy to warp due to the difference in coefficient of thermal expansion between bonded members during soldering. In addition, heat generated from the semiconductor chips passes through the metal/ceramic insulating substrate, solder and base plate to be radiated from the radiating fin or cooling jacket to air or cooling water. Therefore, if the base plate warps during soldering, when the radiating fin or cooling jacket is mounted on the base plate, the clearance therebetween increases, so that the heat sink characteristic of the base plate extremely deteriorates. Moreover, since the thermal conductivity of solder itself is low, it is desired that a power module for allowing the flow of heavy-current has a higher heat sink characteristic. In order to solve these problems, there is proposed a metal/ceramic circuit board wherein a base plate of aluminum or an aluminum alloy is bonded directly to a ceramic substrate without soldering between the base plate and a metal/ceramic insulating substrate (see, e.g., Japanese Patent Laid-Open No. 2002-76551). As a mold for producing such a metal/ceramic bonding substrate, there is proposed a mold in which a cavity (a base plate forming portion) for forming a base plate is formed, the bottom face of the cavity having a plurality of recessed portions (ceramic substrate housing portions) for housing therein ceramic substrates, and the bottom face of each of the recessed portions having a recessed portion (a metal plate forming portion for circuit pattern) for forming a metal plate for circuit pattern (see, e.g., Japanese Patent Laid-Open No. 2005-74434).
However, if a metal/ceramic bonding substrate is produced by means of such a mold in which a cavity or recessed portion corresponding to the shape and size of a metal plate (a base plate or a metal plate for circuit pattern) to be bonded to a ceramic substrate is formed, when a molten metal of aluminum or an aluminum alloy is solidified by cooling the mold after the molten metal is injected into the mold, there are some cases where the amount of the supplied molten metal is insufficient for solidification and shrinkage to easily cause the so-called “shrinkage cavities” and/or “voids” in the metal member of aluminum or the aluminum alloy of the produced metal/ceramic bonding substrate, so that the metal member has an undesired shape to deteriorate the reliability of the metal/ceramic bonding substrate. For example, there are some cases where the corner portions of a base plate are rounder than a desired shape if the base plate is bonded to a ceramic substrate, or where the corner portions of fins are rounder than a desired shape if a base plate with fins are bonded to a ceramic substrate. In addition, if the metal member is made of an aluminum alloy, there are some cases where hot cracks are produced in the metal member in a cooling process to deteriorate the reliability of a metal/ceramic bonding substrate.